Conditioning micronised polymers for rotational moulding

Not all polymers can be directly processed by rotational moulding. Achieving this requires the material (compounding), the powder (micronisation, typically between 60 and 500 µm) and the process to be designed together. This article explains the contribution of each stage, the characteristics the powder must have and how to validate the processing window.

Rotational moulding is a technology particularly suited to manufacturing hollow parts, large components or complex geometries, without the need to apply high pressures during processing [1].

However, these characteristics also mean that not all polymeric materials can be directly processed by rotational moulding. To develop higher value-added applications, it is not enough to select a polymer with specific properties: the material, powder and processing technology must be designed together.

What does each technology contribute?

In this context, combining three technologies is particularly interesting because each one addresses a different question:

Each technology addresses a different question:

Compounding: designing the material according to the application

Compounding makes it possible to combine a polymer matrix with additives, fillers, reinforcements or other polymers to develop materials with properties tailored to the requirements of each application.

This technology can be used to work on, among others, the following functionalities [2]:

• Improved impact resistance and ductility, through impact modifiers and compatibilisation strategies.
• Improved barrier properties, by incorporating fillers capable of hindering the diffusion of gases or liquids [3, 4].
• Modification of rheology and flowability, particularly relevant in a low-shear process such as rotational moulding.
• Improved thermal stability, required to withstand relatively long heating cycles.
• Incorporation of additional functionalities such as thermal or electrical conductivity, fire resistance, UV protection or antimicrobial properties, among others [2].

How can a property be improved without compromising processing?

When developing materials for rotational moulding, an additional challenge arises: the formulation cannot be designed solely according to the final properties.

For example, incorporating impact modifiers, fillers or nanomaterials can significantly affect:

• The viscosity of the molten polymer.
• Crystallisation.
• Thermal stability.
• Phase dispersion.
• Or even the subsequent behaviour of the material during micronisation.

The aim is therefore to find a balance between functionality, processability and final performance.

Micronisation: reducing pellet size is not enough

Once the formulation has been developed, the next step is to physically adapt the material to the rotational moulding process.

Although other options may exist, thermoplastic rotational moulding generally uses materials in powder form. Therefore, the pellet obtained after the compounding process must be conditioned through a size-reduction or micronisation process [1].

However, obtaining a good powder for rotational moulding involves controlling much more than its size.

What characteristics should a suitable powder have?

According to Crawford and Throne [1], powders commonly used in rotational moulding are approximately between 60 and 500 µm, although the optimum distribution depends on the polymer, the grinding process and the final application.

Is finer always better?

Not necessarily.

Reducing particle size increases the material’s specific surface area and can promote faster heating, but an excessive proportion of fine particles can reduce powder flowability and increase its sensitivity to thermal degradation [1].

At the other extreme, excessively coarse particles can:

• Require more time to heat up.
• Delay coalescence.
• Make it more difficult to remove pores.
• Lead to poor surface quality.

The particle size distribution must therefore seek a balance between flowability, packing and thermal behaviour.

Why does particle shape matter?

During rotational moulding, the material must move freely inside the mould before it begins to adhere to its walls. For this reason, particle geometry has a direct effect on its behaviour.

Compact and relatively regular particles promote flow, whereas excessively elongated geometries, tails or highly irregular edges can cause interlocking and reduce the powder’s ability to distribute evenly [1].

The conditions used during micronisation must be optimised to achieve the right combination of size, distribution and morphology. These include:

• Mill configuration.
• Temperature and cooling.
• Feed rate.
• Subsequent screening.

Rotational moulding: jointly validating material, powder and process

The final step is to determine whether the developed and micronised compound can be processed in a stable and reproducible manner.

During the rotational moulding cycle, the powder introduced into the mould goes through different stages [1]:

Therefore, validation should not focus on a single temperature or speed, but on designing a complete processing window.

Three particularly important variables

1. Thermal cycle design

The aim is to provide sufficient energy to achieve good coalescence and densification without degrading the polymer [1].

For this reason, it is particularly useful to analyse not only the oven temperature, but also the thermal profile experienced by the material during the cycle.

2. Rotation speeds and axis ratio

Rotational moulding normally uses movement around two main axes. Speeds are relatively low and the process is not based on centrifugal forces, but on the continuous movement of the powder inside the mould [1].

The combination of the main and secondary axis speeds – and therefore the ratio between them – influences how the material is distributed across the different areas of the mould and, consequently, the uniformity of wall thickness.

There is no universally valid speed ratio: it must be adapted to the geometry of the part.

3. Cooling strategy

The cycle does not end when the polymer has melted.

The cooling rate can affect [1]:

Very rapid cooling can reduce cycle time, but does not necessarily provide the best part. For this reason, the cooling stage must be designed with the same care as the heating stage.

An integrated strategy: from pellet to part

The development of advanced materials for rotational moulding should be approached as a complete technology chain, because a failure at any stage compromises the result:

1. A formulation may offer excellent mechanical or barrier properties and yet not be viable if its viscosity hinders coalescence.
2. A correctly designed compound can cause manufacturing problems if the powder shape or particle size distribution is unsuitable.
3. And a perfectly conditioned powder can result in defective parts if the thermal cycle, rotation speeds or cooling are not properly optimised.

An effective development methodology can therefore be summarised as follows:

This methodology makes it possible to feed information back into each stage: if melting, distribution or consolidation problems are detected during rotational moulding, it may be necessary to modify the particle size distribution or even return to the initial formulation.

From conventional materials to high-value-added rotational moulding

The combination of compounding, micronisation and rotational moulding makes it possible to extend the possibilities of this technology towards increasingly demanding applications.

Development is not limited to selecting a raw material; it makes it possible to design the material specifically for the required functionality and process, opening the door to parts with improved mechanical, barrier, thermal, chemical or functional properties.

How do we work on rotational moulding at AIMPLAS?

At AIMPLAS, we integrate the three stages of the chain:

• Compounding: development of tailor-made compounds, incorporation and dispersion of fillers and nanomaterials, compatibilisation, property modification and process optimisation [2].
• Micronisation: a wide working range, from developments at gram scale to the micronisation of kilogram-scale quantities at semi-industrial scale.
• Rotational moulding: testing and validation of developments and materials, as well as small-batch production, enabling development to be transferred from the initial material through to the final part.

The integration of these capabilities with material conditioning and subsequent processing makes it possible to approach development from the initial formulation through to final part validation, supporting the development of new higher value-added rotational moulding applications.

References

[1] Crawford, R. J.; Throne, J. L. Rotational Molding Technology. Plastics Design Library / William Andrew Publishing, Norwich, New York, 2002. ISBN 1-884207-85-5.

[2] AIMPLAS. Compounding. Plastics Technology Centre. Technology and solutions for the development of new materials through compounding, formulation optimisation, incorporation and dispersion of fillers, fibres, nanomaterials and additives.

[3] Vannini, M.; Marchese, P.; Celli, A.; Marega, C.; Lorenzetti, C. «Strategy to improve PA6 performances by melt compounding». Polymer Testing, 67 (2018), 84-91. DOI: 10.1016/j.polymertesting.2018.02.021.

[4] Dong, C.; Liu, Y.; Li, J.; Bin, G.; Zhou, C.; Han, W.; Li, X. «Hydrogen Permeability of Polyamide 6 Used as Liner Material for Type IV On-Board Hydrogen Storage Cylinders». Polymers, 15(18) (2023), 3715. DOI: 10.3390/polym15183715.